Liquid flow control device

ABSTRACT

A liquid flow control device (20) comprises a variable orifice (28/30) in a conduit (15), and branch lines (41, 42) above and below a flow restriction, which may be the variable orifice itself. The orifice is controlled to maintain a predetermined relation between the pressures (P 1 , P 2 ) in the two branches. A metering valve (60) for a second liquid to be injected into the conduit may be controlled simultaneously to maintain a preset ratio between the flow coefficients of the valve and the orifice, so that if the pressures above and below the metering valve are the same as the pressures above and below the variable orifice, the respective flow rates will stay in the same proportion, and the proportion of second liquid injected into the liquid in the conduit will be constant at all flow rates. The orifice control may be by piston and cylinder (50, 52) driven by the liquid pressures in the branch lines, the moving piston turning an orifice control rod (36) which also rotates a variable plug (62) in the metering valve. The specific application is for the injection of foam concentrate into a fire water distribution system, especially on an offshore oil platform, where flow rates and pressures will vary in use according to the instantaneous demands for water. The metering device is capable of handling water flows between 100 and 2250 m 3  /hr. at pressures of around 15 bar while providing an output solution within about ±10% of the 1%, 3% or 6% target concentration over the full range of pressure and flow conditions, with the ability to withstand surge pressures as high as 30 bar.

This invention relates to a liquid flow control device. In a preferredembodiment the invention is concerned with a device for meteringcontrolled amounts of one liquid into another liquid flowing through aconduit at high pressures and flow rates.

The preferred device according to the invention finds particular use incontrolling the injection of a surface-active agent into a fire waterdistribution system, to produce a foam when the water is released into afire zone at atmospheric pressure. A typical class of agents is known asAFFF, aqueous film-forming foam, and is injected as a concentrate intothe fire water to give a concentration of AFFF in the resultant solutionof 1%, 3% or 6% according to operational requirements and the nature ofthe particular AFFF additive used.

On an offshore oil or gas platform the AFFF is injected into sea waterfor firefighting purposes, and the quantities of water required to bedelivered by the firepumps can typically vary from 100 to 2250 m³ /hr.at pressure of around 15 bar. Flow rates and pressures will however varyin use according to the instantaneous demands for water being made onthe system, and it is necessary to vary the injection rate in order tomaintain the correct AFFF concentration in the water in spite of thesefluctuations.

The currently used method of proportioning the AFFF to the waterrequires a main venturi to be installed in the conduit supplying firewater to the ring main on the platform, and a second venturi to beinstalled in the AFFF injection line. The AFFF is injected into thewater at the throat of the main venturi. The AFFF is supplied to thesecond venturi at the same pressure as the water is supplied to the mainventuri. The two venturis have flow coefficients in a fixed ratio, sothat the ratio of AFFF introduced into the water at the throat of themain venturi is in the required proportion to the volume of waterflowing. The disadvantages of this method include high pressure losseswhen working at high flow rates; low pressure losses at low flow rates,leading to inaccurate proportioning; and the readily available equipmenthaving a maximum flow rate capability at the present time of only 1000m³ /hr.

The present invention provides a flow control device with differentcharacteristics, enabling some or all of these disadvantages to bemitigated.

In one aspect, the invention provides a flow control device forcontrolling liquid flow through a conduit, comprising a variable orificein the liquid flow path in the conduit, a first branch line for liquidconnected to the conduit upstream of liquid flow restriction means inthe conduit, a second branch line for liquid connected to the conduitdownstream of said liquid flow restriction means, and orifice controlmeans operatively connected to the variable orifice and responsive tothe liquid pressures in the first and second branch lines to vary theorifice to maintain a predetermined relation between the pressures inthe said lines.

The orifice control means may comprise piston and cylinder means adaptedto be driven by the liquid pressures in the first and second branchlines. The predetermined relation between the pressures in the saidlines may for example be a pressure ratio, a pressure difference, or acombination of these, and will normally be determined by the design ofthe orifice control means.

The restriction may be a fixed restriction, such as a fixed orificeplate, downstream of the variable orifice, in which case the device canfunction as a flow regulator valve to maintain a constant liquid flowrate through the restriction. However, in the preferred application ofthe device, the restriction is the variable orifice itself, and thedevice then varies the flow coefficient of the orifice in order tomaintain a predetermined relation between the pressures on either sideof the variable orifice. Accordingly, in a second aspect, the inventionprovides a device for metering a second liquid into a liquid flowingthrough a conduit, comprising a flow control device as aforesaid,wherein the restriction is the variable orifice, a supply line for thesecond liquid leading into the conduit downstream of the variableorifice where the pressure is substantially the same as the pressure inthe second branch line, and a variable metering valve in the supply linefor metering the second liquid, wherein the metering valve is suppliedwith the second liquid at substantially the same pressure as thepressure in the first branch line and is adapted to be driven by theorifice control means to vary its flow coefficient in a predeterminedratio to the varying flow coefficient of the variable orifice.

In such a metering device, the pressure drop is related to the liquidsupply pressure in the conduit, rather than to the liquid flow rate. Ithas been found that such a device can deliver higher liquid flows thanare at present readily available with venturi-based devices.

In order to meter a substantially constant proportion of the secondliquid into the first, the variable metering valve is configured tomaintain a substantially constant ratio between the flow coefficients ofthe metering valve and the variable orifice, when both are driven by theorifice control means.

The invention will now be further described with reference to theaccompanying diagrammatic drawings, not to scale, which show oneembodiment of the invention by way of example only.

In the drawings:

FIG. 1 is a diagram of a typical working environment for a device formetering a second liquid into a first liquid flowing through a conduit,in this case a foam proportioning device in an offshore platform firewater system;

FIG. 2 is a sectional view of the device, seen from one side;

FIG. 3 is a front elevation of a fixed orifice plate used in the device;and

FIG. 4 is a front elevation of a movable orifice plate used in thedevice.

FIG. 1 shows a main platform water ring main 11 for supplying fire waterto a variety of hydrants 12, sprinklers 13 and process area delugesystems 14. The main is normally pressurised at about 8 bar with seawater supplied through conduit 15 by jockey pump 16 from sea water inletpipe 17.

When the pressure in the ring falls because of fire water demand, mainfire pump 18 is activated and pumps sea water from inlet pipe 19 at apressure of about 15 bar through foam proportioning unit 20 into conduit15.

The foam proportioning unit 20 is supplied with liquid foam concentrate(AFFF) from reservoir 21 by pump 22 through shut-off valve 23 in foaminlet line 24. Provided that valve 23 is open, the proportioning unit 20meters the foam concentrate into the main water supply conduit 15,maintaining the required concentration of 1%, 3% or 6% in spite of thevariations in water flow through the conduit.

The metering device of the invention is used as the foam proportioningunit, and is shown in detail in FIG. 2

The device 20 is located in the main conduit 15, which may be about 400mm in diameter. Water is pumped through in the direction of the arrows.The device includes a cylindrical housing 22 enclosing an inlet collar25, and an outlet collar 26 which carries a fixed orifice plate 28across the conduit. A second orifice plate 30 is rotatably mounted on acentral pin 32 on the upstream face of the fixed orifice plate, betweenthe inlet collar and the outlet collar. The rotation of this secondplate is controlled by a rod 36 extending diametrically across thehousing 22 in front of the upstream face of the plate and extendingbeyond the housing through seals 37, 39.

FIG. 3 shows the fixed orifice plate 28, with six generally triangularflow passages 34 and the central mounting pin 32 for the movable plate30.

The movable orifice plate 30 and associated control rod 36 are shown inFIG. 4. The movable plate is substantially identical in form to thefixed plate 28 on which it is mounted, but is provided with two pins 38projecting from its upstream face. The rod 36 is provided with aneccentric portion 40 which is located between the pins 38, so that axialrotation of the rod causes the eccentric to rotate the plate 30 over theplate 28. The plates 28 and 30 together constitute a variable orificeplate. The arrangement is such that at one extreme the flow passages inthe two plates are aligned, giving a free effective area for water flowof 50-60%, typically 54%, of the conduit's area, and at the otherextreme the flow passages are offset, giving a free effective area ofonly 0-10%, typically 4%.

The housing 22 and the inlet collar 25 are tapped at a distance of aboutone conduit diameter d upstream of the variable orifice plate 28/30,where the liquid pressure is P₁, to supply two branch lines 41, 44 atthat pressure. Similarly, the housing 22 and the outlet collar 26 aretapped at a distance of about half one conduit diameter downstream ofthe variable orifice plate 28/30, where the liquid pressure is P₂, tosupply to branch lines 42, 46 at that pressure.

Tappings 41 and 42 provide liquid at pressure P₁ and P₂ respectively tocontrol the rotation of orifice plate control rod 36. This is achievedin the illustrated embodiment of the invention by taking pressure P₁ topiston and cylinder assembly 50 and pressure P₂ to piston and cylinderassembly 52. The two assemblies share a common piston rod 54 which islinked to orifice control rod 36 by, for example, a rack and piniondrive.

The effects on the liquid pressures in the conduit will depend on thedetailed design. For example, the relation between the pressures P₁ andP₂ at equilibrium will be determined by the relative effective workingareas of the pistons in the piston and cylinder assemblies 50 and 52,and any other forces which may be applied to the piston and cylinderassemblies. In this example, the area of cylinder 50 is less than thatof cylinder 52, which results in a predetermined differential beingmaintained between P₁ and P₂.

In operation, the relation between P₁ and P₂ is maintained, in spite offluctuations in the water flow through the conduit 15, becausevariations cause the piston rod 54 to move, driving the orifice controlrod 36 and varying the orifice 28/30 until the relation isre-established.

On the opposite side of the housing 22 is a variable metering valve 60for liquid foam concentrate. The valve 60 comprises cylindrical chamber61 containing a rotary plug 62 mounted on the further end of the orificecontrol rod 36. The chamber 61 has an outlet connected to the branchline 46 and an inlet 64 connected to the foam concentrate inlet line 24by way of a pilot operated regulator valve 66, which is controlled bywater at pressure P₁ in branch line 44 to admit the liquid foamconcentrate from the line 24 to the inlet side of the chamber 61 at thesame pressure P₁. The outlet side of the chamber is at pressure P₂, thepressure in the branch line 46.

The rotary plug 62 is provided with a reduced radius portion 68 overpart of its circumference. Liquid foam concentrate can only flow fromthe chamber inlet to the chamber outlet over this portion, between theplug and the inner wall of the chamber. The amount of liquid that flowsis regulated by rotation of the plug, constraining the liquid to flowthrough a narrower or wider gap. The movement of the plug is directlylinked to the movement of the variable orifice plate 28/30 by thecontrol rod 36. Liquid flows through the metering valve 60 at the samepressure differential, P₁ -P₂, as the liquid flowing through thevariable orifice. The absolute value of P₁ -P₂ is typically limited towithin the range 0.35 to 0.75 bar.

The dimensions of the reduced radius portion 68 are previously matchedto the variable orifice so that the flow coefficients of the meteringvalve and the variable orifice are in a substantially constant ratioover their full ranges of movement, the ratio typically being chosen togive a 1:99, 3:97 or 6:94 proportion of liquid foam concentrate to waterdownstream of the metering device at all flow rates. The proportion offoam can readily be changed by replacing one rotary plug 62 with anotherhaving a differently configured reduced radius portion 68.

A metering device of this kind is capable of handling water flowsbetween 100 and 2250 m³ /hr. while providing an output solution withinabout ±10% of the 1%, 3% or 6% target concentration over the full rangeof pressure and flow conditions, with the ability to withstand surgepressures as high as 30 bar.

Although the invention is described herein in terms of specific elementsand combinations of elements, it is envisaged that each element may becombined with any other or with any combination of other elements. Theinvention is not limited only to the particular combinations of elementsrecited, which may not be indispensable to the invention; alternativesmay be employed. The invention lines in all matters disclosed and is notnecessarily limited to the specific statements of the invention putforward herein.

I claim:
 1. A device for metering a second liquid into a first liquidflowing through a conduit, comprising: a variable, flow controllingorifice in the liquid flow path in the conduit; orifice control means,responsive to the liquid pressures in the liquid flow path at locationsspaced apart from and, respectively, upstream and downstream of thevariable orifice, for varying the orifice to maintain a predeterminedrelation between the said liquid pressures at the said upstream anddownstream locations; a supply line for the second liquid leading intothe conduit downstream of the variable orifice at a position adjacentthe said downstream location and where the liquid pressure issubstantially the same as the liquid pressure at the said downstreamlocation; and a variable metering valve in the supply line for meteringthe second liquid; wherein the metering valve is supplied with thesecond liquid at substantially the same pressure as the liquid pressureat the said upstream location and is adapted to be driven by the orificecontrol means to vary its flow coefficient in a predetermined ratio tothe varying flow coefficient of the variable orifice.
 2. A meteringdevice according to claim 1 comprising a first branch line connected tothe conduit at the said location upstream of the variable orifice and asecond branch line connected to the conduit at the said locationdownstream of the variable orifice, and wherein the orifice controlmeans is respective to the pressures of the first liquid in said branchlines.
 3. A metering device according to claim 2 wherein the orificecontrol means comprise piston and cylinder means adapted to be driven bythe liquid pressures in the first and second branch lines.
 4. A meteringdevice according to claim 2 wherein the first branch line is connectedto the conduit at a distance of about one conduit diameter upstream ofthe variable orifice, and the second branch line, and the supply linefor the second liquid, are connected to the conduit at a distance ofabout half one conduit diameter downstream of the variable orifice.
 5. Ametering device according to claim 1 wherein the variable metering valveis configured to maintain a substantially constant ratio between theflow coefficients of the metering valve and of the variable orifice,when both are driven by the orifice control means.
 6. A metering deviceaccording to claim 5 wherein the variable metering valve comprises achamber having an inlet and an outlet and containing a rotary plugprovided with a reduced radius portion over part of its circumferencedefining a flow path from the inlet to the outlet between the plug andan inner wall of the chamber, so that rotation of the plug constrainsthe liquid to flow along a narrower or wider portion of the flow path,and the dimensions of the plug are matched to the variable orifice sothat the flow coefficients of the metering valve and of the variableorifice are in a substantially constant ratio over their full ranges ofmovement.
 7. A metering device according to claim 1 wherein the variableorifice comprises a fixed orifice plate extending across the conduit, asecond orifice plate rotatably mounted on the upstream face of the fixedorifice plate, a plurality of flow passages in the two plates, and anorifice control rod provided with an eccentric portion located betweentwo projections from the upstream face of the second orifice plate,whereby axial rotation of the rod causes the eccentric to rotate thesecond plate over the fixed plate and thereby vary the alignment of theflow passages in the two plates.
 8. A metering device according to claim7 wherein the orifice control means comprise a common piston rodcarrying a first piston driven by liquid from the first branch line anda second piston drive in an opposite sense by liquid from the secondbranch line, the piston rod being linked to the orifice control rodwhereby axial movement of the piston rod rotates the orifice controlrod.
 9. A metering device according to claim 8 wherein the first andsecond pistons are of different effective areas.
 10. A metering deviceaccording to claim 8 wherein the rotary plug is driven by the orificecontrol rod whereby rotation of the plug is linked to rotation of thesecond plate of the variable orifice.
 11. A metering device according toclaim 1 wherein the metering valve comprises a pilot operated regulatorcontrolled by liquid supplied from the conduit upstream of the variableorifice where the liquid pressure is substantially the same as theliquid pressure at the said upstream location, to admit the secondliquid to the inlet of the valve at substantially the same pressure. 12.A fire water distribution system comprising a water supply conduit and ametering device according to claim 1 for metering a solution of asurface-active agent into the water in the conduit.
 13. A device formetering a second liquid into a first liquid flowing through a conduit,comprising:a variable, flow controlling orifice comprising a fixedorifice plate extending across the conduit, a second orifice platerotatably mounted on the upstream face of the fixed orifice plate, aplurality of flow passages in the two plates, and means for rotating thesecond plate over the fixed plate thereby varying the alignment of theflow passages in the two plates; orifice control means connected to themeans for rotating the second plate, and responsive to the liquidpressures in the liquid flow path at locations spaced apart form and,respectively, upstream and downstream of the variable orifice forvarying the orifice to maintain a predetermined relation between thesaid liquid pressures at the said upstream and downstream locations; asupply line for the second liquid leading into the conduit downstream ofthe variable orifice at a position adjacent the said downstream wherethe liquid pressure is substantially the same as the liquid pressure atthe said downstream location; and a variable metering valve in thesupply line for metering the second liquid; wherein the metering valveis supplied with the second liquid at substantially the same pressure asthe liquid pressure at the said upstream location and is adapted to bedriven by the orifice control means to vary its flow coefficient in apredetermined ratio to the varying flow coefficient of the variableorifice.